Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Lincoln Illinois
819 PM CST Thu Dec 12 2013
issued 816 PM CST Thu Dec 12 2013
Have lowered low temperatures in the southeast as snow cover,
light winds, clear skies, as allowed temps to plummet. Elsewhere,
only minor grid tweaks are needed.
issued 524 PM CST Thu Dec 12 2013
High pressure continues to retreat to the southeast this evening.
A cold front is expected to stall out northwest of the terminal
sites Friday. Winds should be generally at 160-200 degrees through
the taf valid period as area remains in the warm sector.
Strong northern stream jet continues to roar across the
Midwest/Great Lakes and will translate slowly eastward over the
period. However, the proximity of the right entrance region of the
jet suggests that periods of mid-level cloud through early
Friday. By Friday afternoon, open wave remnant of closed low
currently over Desert Southwest will approach the area and provide
good moisture flow off the western Gulf of Mexico. Will introduce
MVFR vsbys in -sn late in the period at all but kcmi as NAM time
heights indicate rapid saturation of the column as models hint at
significant lift with coupled jet area nearby. Sref from 15z
suggests pcpn may begin as a mix of frozen and liquid at kspi and
kdec but will keep all frozen for now since it will starting late
and hone precip type in 06z issuance.
issued 335 PM CST Thu Dec 12 2013
A quick update at 330 PM...after coordination with surrounding
offices... we will be issuing a Winter Weather Advisory this
afternoon for all of our area for Friday afternoon into Saturday
morning. Snow totals will be mainly 3 to 5 inches north of I-70
and around 3 inches total south of I-70, with possible sleet and
freezing rain early on in that area. This update to add coding
for a Winter Weather Advisory at the end of the afd.
Previous discussion... the main focus for the short term is the
storm system for Friday afternoon into Saturday. Snow amounts
and precip type issues lead the way. The latest model runs
continue to show variation in the mesoscale details, but the
overall pattern of broad isentropic lift and higher moisture
content should lead to a snowy end to the week. Some sleet-
freezing rain issues remain for areas south of I-70, with a mix of
sleet-snow possibly in our western areas as far north as I-72.
Warmer air surging into Missouri has pushed some highs near 43, in
areas with no snow cover. Air that warm would raise the potential
for all rain in our S-SW areas for a time Friday afternoon. A
return of cold air Friday night should help to diminish that
potential, but that scenario would lower snow totals at the very
Short term...tonight through Saturday night.
Warmer air returning to the area today on the back side of the
surface ridge will help keep low temps tonight about 15 to 18
degrees warmer than last night...with lows generally in the teens.
Steady south winds will add a chill to the air.
Friday will see our storm system take shape. The primary surface
low is forecast to deepen over northern Texas as a warm front
develops to the NE of the low into central Illinois. That warm front
will merge with an advancing cold front from the north to set up a
baroclinic zone across our forecast area. Isentropic lift will
intensify Friday afternoon helping to saturate the lower boundary
layer from west to east across the middle of our forecast area
from Rushville toward bmi and cmi. Farther south and north of that
axis, precipitation will be less intense, with precip type issues
to the south. Forecast soundings indicate an initial period of
sleet may develop from Jacksonville to Springfield and southeast
to Effingham. As previously mentioned, if the warm air surge south
of the warm front is as strong tomorrow as today across Missouri,
a period of change to all rain may even develop along and south of
that line. We are leaning toward a colder solution and a wintry
mix as the more likely scenario. That is mainly due to the
expectation that colder air will continue flow south into our
forecast area despite the surface low not moving into Arkansas
until 12z Saturday.
The stalled frontal boundary and gradual rotation of the
baroclinic zone over central Illinois complicates the precip type
solution, but feeling is for snowfall as the primary-type along
and north of I-72 in the first 12 hours of the event, where the
highest precip amounts are expected. We have the potential for
periods of banded snow to develop in the stationary regions of
isentropic lift and coupled-jet dynamics. That could push snowfall
totals toward 6 inches across Schuyler County and toward McLean
County. The axis of heavier snows has shifted with each model run,
the most recent change being a northward shift from the I-72
corridor toward I-74. Mesoscale features will likely develop in
the synoptic scale forcing. For now we will not issue a watch for
6+ inches of snow, with the expectation that snow amounts will
remain in the advisory range of 3-5 inches along and north of
South of I-72, some sleet or freezing rain would reduce snow
totals through Friday evening, but a secondary wave of forcing for
precip will lift from SW to NE across southeast Illinois from midnight
to noon on Saturday. Enough cold air should be in place for that
precip to be all snowfall, which could accumulate up to 3 inches
across our southeastern counties in the last half of the event.
Snow will shut down Saturday afternoon as the low pulls into the
Tennessee Valley. Colder air will surge into the area on northwest winds,
which may blow and drift the snow Sat afternoon, especially west
of I-55 where winds will increase sooner.
Saturday night should see clouds lingering with sustained west-northwest winds
in the 10 to 15 mph range as pressure rises continue ahead of
advancing high pressure. The cold advection will cause lows to dip
into the teens, after lows on Friday night in the mid to upper
Long term...Sunday through Thursday.
Dry conditions are expected for the end of the weekend and much of
next week as the storm track remains north of Illinois. A glancing blow
of snow may reach as far south as northern Illinois on Tuesday and tues
night, but our areas should remain dry. We should see highs for
much of the week approach the freezing mark in the north and mid
to upper 30s south as the core of cold air remains across the
Winter Weather Advisory from 3 PM Friday to noon CST Saturday
Winter Weather Advisory from 3 PM Friday to 6 am CST Saturday